1. Field of the Invention
The present invention relates to an antenna, and in particular to a planar Inverted-F antenna (PIFA) employed in an electronic device and a method of making the same.
2. Description of the Prior Art or Related Art
Microstrip antennas can be applied as built-in antennas for many kinds of portable electronic devices for their small and compact structure. The resonant frequency of a traditional microstrip antenna usually is determined by the size of antenna's radiating element. Typically the length of the radiating element is a half of the radiating wavelength of the operating frequency.
For portable application, specially as a built-in antenna, a planar Inverted-F antenna (PIFA), especially a printed PIFA antenna is more preferred design to get smaller antenna for its operation length only being ¼ to the wavelength of the operating frequency. Specially, inserting a dielectric substrate with a high dielectric constant between the radiating element and ground portion will get a shorter operating length of the antenna (i.e. less than 1/10 wavelength of operating frequency). When operating frequency and dielectric substrate are determined, the length of the radiating element is substantially decided. To fully utilize the space of a portable device, the radiating element can be curved in one or more surface of the dielectric substrate. For example, the radiating element a traditional printed PIFA is typically a straight trace. The antenna will become shorter when a U-shaped (or other spiral shape) radiating element is introduced rather than a straight one.
U.S. Pat. No. 6,535,443 discloses a printed PIFA antenna with a spiral-radiating element. This PIFA antenna comprises a printed circuit board (PCB) 310, a dielectric substrate 320 disposed on a PCB 310 and a spiral metal strip 315 acting as a radiating element printed on a top surface the substrate 320. A matching bridge 330 shorts the strip 315 to the PCB 310. The antenna feed pin 325 disposed on the side surface connects with the strip 315.
However, the spiral metal strip is only disposed one of the surfaces of the substrate. The inner space of the substrate is not used. If the inner space is used to receive the printed radiating element the antenna structure will be more compact.
Hence, an improved antenna assembly is desired to overcome the above-mentioned disadvantages of the prior and related arts.